About 130,000 anterior cruciate ligament (ACL) reconstructions are performed each year in the U.S. These repairs involve repairing tears in the knee meniscus. The standard treatment is to leave asymptomatic damage alone or perform partial or total meniscectomies (removing damaged meniscal tissue). Both solutions increase the risk of developing osteoarthritis and the eventual need for a total knee replacement. Meniscal allograft research has given mixed results and no long-term data, leaving meniscal allografts as a procedure that few surgeons perform. Nonetheless, healthy joints to have intact and functional menisci to prevent osteoarthritis.
One of the difficulties in creating artificial and tissue-engineered menisci is the complex structure-function relationship of the native tissue. The meniscus is a biphasic tissue composed of interstitial fluid and a matrix composed of predominantly circumferential collagen fibers and glycosaminoglycan-rich proteoglycans. The hierarchical structure and surface network of randomly oriented collagen fibers have a highly aligned circumferential fibers throughout, which enables its mechanical function. This collagen organization transfers compressive loads into tensile hoop stress. These collagen fibers provide the tissue's mechanical anisotropy with the tensile elastic modulus one to two orders of magnitude larger than the compressive elastic modulus.
Conventional hydrogels lack the resistance to fatigue and fracture under continue cyclic loading or extensive compressive strain required for biomedical implants. Mechanical performance in hydrogels ranges from very soft and brittle gels to extremely tough and stiff gels, all of which may be widely applied. Conventional hydrogels, such as highly crosslinked polyvinyl alcohols, have been particularly successful in low-load bearing biomedical applications including drug delivery, wound dressings, and injectable fillers, due to their high water content and biocompatibility. Thus, there exists a need for new hydrogels that can be used to form soft tissue mimetics, such as knee meniscus replacements.